What are the valid characters of various variables in C language?

Internal: For example, a variable name that can be controlled by a language compiler can allow the use of symbols as long as 3 1 bit. As long as the first 3 1 bits of two names are different, they are two identifiers.

External: for example, file name, which is not controlled by language and is related to the existing machine environment. The format of DOS era is 8.3, Windows can be 255, and the situation of various Unix is different.